Process for coating cellulosic substrates



United States Patent O 3,288,630 PROCESS FQR COATING CELLULOSIC SUBSTRATES Mark Plunguian, Newark, Del., assignor to Hercules Incorporated, a corporation of Delaware No Drawing. Filed Feb. 23, 1965, Ser. No. 434,682 Claims. (Cl. 11762) This application is a continuation-in-part of my copending application Serial No. 391,321, filed August 21, 1964, now abandoned, which in turn was a continuation-in-part of copending application Serial No. 220,535, filed August 30, 1962, now US. Patent 3,198,645.

The present invention relates broadly to a novel process for providing a cellulosic substrate with a heat-scalable nitrocellulose coating of great heat seal strength. In a more limited aspect, the present invention relates to a novel process for providing paper with a heat-scalable nitrocellulose coating of great heat seal strength during the papermaking process.

In copending application Serial No. 391,321 there are described novel nitrocellulose coating compositions which can be satisfactorily employed to coat paper during the papermaking process by application of the coating composition in the size press of the papermaking machine. These compositions are fluid dispersions of nitrocellulose particles and plasticizer in aqueous medium in which at least a major portion of the plasticizer exists in a phase separate from the nitrocellulose phase of the dispersion. Paper coated with the compositions of my aforesaid copending application in the size press of a papermaking machine can be satisfactorily passed over the hot drying cans of the papermaking machine without sticking to the drying cans, and the resulting dried coated paper can be heat sealed by conventional heat sealing means.

Although the strength of the heat seal obtainable with the compositions of copending application Serial No. 391,321 is adequate for many purposes, particularly with heavier coatings above about 1.5 pounds per 3,000 sq. ft., and especially after hot calendering the dried coatings, it is inadequate for some applications, notably in the packaging art, where lighter weight coatings below about 1.5 pounds per 3,000 sq. ft. are desired for economic reasons. In the packaging art a very strong heat seal is desired, so strong in fact, that when an attempt is made to separate the heat-sealed joint the paper is torn. Prior to the present invention paper tear heat-seal strengths have been difficult to obtain with the compositions of copending application Serial No. 391,321, except in some instances with heavier coating above about 1.5 pounds per 3,000 sq. ft., and especially after hot calendering.

It is an object of the present invention, therefore, to provide a method for improving the heat-seal strength of nitrocellulose coatings derived from the compositions of copending application Serial No. 391,321 to the point where the paper is torn when an attempt is made to separate the heat-sealed joint, even in lighter weight coatings below about 1.5 pounds per 3,000 sq. ft., with or without hot calendering.

Another object of the present invention is to provide a method for obtaining paper tear heat-seal strength of any nitrocellulose coating derived by application of a fluid dispersion of nitrocellulose particles in aqueous medium to paper, even when such dispersions contain little or no nitrocellulose plasticizer.

A further object of the present invention is to provide a method for coating a cellulosic substrate with a heatsealable nitrocellulose coating of great heat-seal strength, said coating being derived by application to the cellulosic substrate of a fluid dispersion of nitrocellulose particles in aqueous medium.

3,288,630 Patented Nov. 29, 1956 A still further object of the present invention is to provide a method for coating paper during the papermaking process with a heat-scalable nitrocellulose coating of paper tear heat-seal strength. Other objects will be apparent from the detailed description that follows.

These objects and others are accomplished in accordance with the present invention which, generally described, comprises applying a coating of a fluid dispersion of nitrocellulose particles in aqueous medium to a cellulosic substrate, drying the applied coating, and subsequently treating the applied coating of nitrocellulose with a separate application of nitrocellulose plasticizer to impart improved thermoplasticity, film continuity, and heat-seal properties to the nitrocellulose coating.

In a preferred embodiment of the invention, paper is coated during the papermaking process by continuously applying a coating of a fluid dispersion of nitrocellulose particles in aqueous medium to said paper at the size press of the papermaking machine, continuously passing the thus coated paper over the heated drying cans of the papermaking machine to dry the applied coating of nitrocellulose and thereafter treating the dried nitrocellulose coating with a separate application of nitrocellulose plasticizer. The amount of said separate application of nitrocellulose plasticizer generally will be between about 0.1 pound and about 0.6 pound of plasticizer, preferably between about 0.2 pound and about 0.4 pound, per 3,000 sq. ft. of nitrocellulose coating, and sufficient to impart improved thermoplasticity, film continuity, and paper tear heat-seal strength to the nitrocellulose coating.

Aqueous dispersion of nitrocellulose particles suitable for practice of this invention may be prepared by a variety of methods. For example, in accordance with US. Patent 2,843,582 to Voris, nitrocellulose is dissolved in a substantially water-immiscible volatile solvent which boils below the boiling point of water, and the resulting lacquer solution thus formed is emulsified, in an aqueous phase consisting of water and an emulsifying agent to form a lacquer-in-water emulsion. A thin film 'of the emulsion is then formed on a surface heated to an elevated temperature below the boiling point of water, and the volatile solvent is evaporated from the emulsified lacquer to precipitate the nitrocellulose in finely-divided form in the water of the emulsion. By controlling temperatures and feed rates during the evaporation, it is possible by this method to produce smooth, fluid, pourable dispersions or slurries containing as high as 50% by weight of finely-divided nitrocellulose particles in water. Nitrocellulose particles prepared in accordance with this patent are tiny spheres and spheroids having a particle size distribution between 1-5 microns in size averaging 2 microns. It is disclosed that the addition of 0.1% to 0.5%, based on solids, of high viscosity methyl cellulose to the dispersions decreases a tendency for the particles to settle out on standing.

Another method for preparing aqueous dispersions of nitrocellulose particles suitable for use in this invention is disclosed in US. Patent 2,722,528 to Johnson. In accordance with this patent nitrocellulose is dissolved in a substantially water-immiscible volatile solvent which boils below the boiling point of water, and the resulting lacquer solution thus formed is emulsified in an aqueous phase consisting of water and an emulsifying agent to form a lacquer-in-wateremulsion. A stream of the resulting emulsion is then passed into a vessel containing an agitated body of hot water maintained at a temperature between and C. to vaporize the solvent and precipitate the nitrocellulose in finely-divided form in the body of hot water. Particles produced by this method are in the form of regular, hollow spheroids having an average particle size of 2 to 10 microns with no particles larger than 15 microns in diameter. The resulting dispersions of finely-divided nitrocellulose particles in water may be adjusted to any desired water content by conventional water removal methods to produce smooth, fluid, pourable dispersions of finely-divided nitrocellulose particles in water, and these dispersions may be stabilized against a tendency for the particles to settle out by addition of a small amount of a water-soluble protective colloid such as methyl cellulose.

Another method for preparing aqueous dispersions of nitrocellulose particles for use in the present invention is disclosed in US. Patent 2,843,583 to Voris. In accordance with this patent, nitrocellulose is dissolved in a lowboiling substantially water-immiscible solvent, and the resulting lacquer solution is emulsified in an aqueous phase consisting of water and an emulsifying agent to form a lacquer-in-water emulsion which is then sprayed into a steam atmosphere to flash evaporate the solvent and precipitate the nitrocellulose in the form of solid spheres and spheroids having diameters not exceeding about microns in the water of the emulsion droplets. It is disclosed that when hydrosol compositions are desired, the water suspension of nitrocellulose particles can be employed directly, after making what-ever adjustment is necessary or desirable in water content of the suspension for proper application consistency.

However, for reasons of economy I prefer to prepare aqueous dispersions of nitrocellulose particles suitable for use in the present invention by the method described in my copending application Serial No. 220,535, filed August 30, 1962. In that application it is disclosed that fibrous nitrocellulose, obtained by nitrating natural cellulose fibers, can be comminuted to a particle size which is eminently suitable for the preparation of useful dispersions thereof in nonsolvent media. These finely divided nitrocellulose particles are prepared by subjecting nitrocellulose fibers suspended in a nonsolvent medium of the group consisting of water, straight and branched chain alkanols having 1-4 carbon atoms, and mixtures thereof, to repeated crushing impacts, as by ball milling, until substantially all of the nitrocellulose fibers have been fractured and comminuted into irregularly shaped fragments, a substantial majority of which have a distribution of particle sizes in the range from sub-micron up to about 1-2 microns in all three dimensions, with no fragments larger than about 3 microns in at least one dimension. Microscopic examination may show a small percentage, usually less than about 5% of tiny needle-like fragments which are less than 3 microns in two dimensions, but greater than 3 microns but not exceeding about microns in length. These do not interfere with the suitability of the comminuted nitrocellulose for the purposes of the present invention. Microscopic examination may also show that some of the comminuted material may be in the form of irregular platelets about 2-3 microns in thickness, but more than 3 microns in diameter. These platelets appear to be full of cracks, and can be broken up by passage through a homogenizer. It appears that these platelets are aggregates of finely comminuted particles of nitrocellulose held together by some physical attraction or force. However, the presence of a small amount of comminuted nitrocellulose in this platelet form does not interfere with the suitability of the comminuted nitrocellulose for the purposes of the present invention.

When water or an aqueous mixture of water and alkonal is employed in the process of copending application Serial No. 220,535 as the nonsolvent medium in which the nitrocellulose is comminuted, the resulting dispersion of finely divided nitrocellulose particles in said nonsolvent medium is concentrated by draining off excess nonsolvent grinding medium my filtration or equivalent means thereby producing a stiff, nonflowing filter cake of finely divided nitrocellulose particles moist with aqueous nonsolvent grinding medium, and containing from about 40% to about 50% by weight of nitrocellulose. This moist cake, upon incorporating thereinto a small and effective amount between about 0.5% and about 5% by Weight of hydrophilic protective colloid, based on weight of nitrocellulose, deflocculates to produce a smooth, fluid, pourable dispersion of the finely divided nitrocellulose particles in the nonsolvent medium present in the moist filter cake, which dispersion is relatively stable with respect to any appreciable tendency for settling out of the nitrocellulose particles on standing; that is, the particles of nitrocellulose remain substantially uniformly suspended in the dispersion for lengthy periods of storage time.

It will be seen from the foregoing description that the nitrocellulose particles prepared by the several methods described are in a very finely divided state of subdivision. This is a necessary characteristic of such particles in order to prepare aqueous dispersions which are smooth and fluid, which have a high nitrocellulose content by weight, and in which the particles of nitrocellulose remain uniformly suspended for relatively lengthy periods. In general, any fluid dispersion of nitrocellulose particles in aqueous medium in which a substantial majority of the nitrocellulose particles are less than about 5 microns in all dimensions, with substantially no particles larger than about 15 microns, is suitable for the purposes of the present invention. The physical shape of the finely divided nitrocellulose particles is relatively unimportant, and, as shown hereinabove, the particles may be in the form of tiny spheres and spheroids which may be solid and/ or hollow, or they may be in the form of irregularly shaped fragments of nitrocellulose fibers.

Smooth, fluid dispersions containing up to about 50% by Weight of these finely-divided nitrocellulose particles in aqueous medium have been prepared, and can be applied as uniform coatings on a cellulosic substrate such as paper or cloth by a variety of conventional application techniques. For example, these dispersions may be applied by roller coating, by means of a doctor blade,

' trailing blade, or air knife, by spraying, and the like. In

preferred embodiments of this invention, the smooth, fluid dispersions of finely divided nitrocellulose particles in aqueous medium are applied continuously to paper in the size press of a papermaking machine.

Finely divided nitrocellulose particles suitable for the purposes of the present invention can be prepared from any of the commercial types and grades of nitrocellulose, having nitrogen contents from about 10.9% to about 13.5% nitrogen by Weight, and of any viscosity characteristic from the very low viscosity 1O centipoise type to exceedingly high viscosity types as exemplified by dynamite grade nitrocellulose. In preferred embodiments, fibrous nitrocellulose, obtained by nitrating natural cellulose fibers, such as cotton, purified cotton linters, purified wood pulp, and the like, in such forms as picked linters, shredded wood pulp, fluifed bulk fibers, finely ground or cut fibers, cubed or granulated fiber aggregates, and the like, are employed.

The essential ingredients of the fluid dispersions of nitrocellulose particles in aqueous medium of this invention are, of course, nitrocellulose and water, and dispersions containing only these two essential ingredients are suitable and operative for practice of this invention, This does not preclude the presence of other ingredients, and, in fact, other ingredients will normally be present. It has already been mentioned that a small amount of a hydrophilic protective colloid can be desirably employed in the dispersions as a deflocculating and/or suspending aid for keeping the finely divided nitrocellulose particles uniformly dispersed, and to inhibit settling out of the nitrocellulose particles during lengthy periods of storage and substantially any hy drophilic protective colloid is suitable for these purposes. Typical hydrophilic protective colloids include, by way of example, water-soluble alkyl ethers of cellulose, hydroxyalkyl ethers of cellulose, mixed alkyl hydroxyalkyl ethers of cellulose, such as methyl cellulose, ethyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyisopropyl cellulose, methyl hydroxyethyl cellulose, methyl hydroxypropyl cellulose, ethyl hydroxyethyl cellulose, ethyl hydroxypropyl cellulose, and the like; water soluble alkyl ethers of starch, hydroxyalkyl ethers of starch, mixed alkyl hydroxyalkyl ethers of starch, such as methyl starch, ethyl starch, hydroxyethyl starch, hydroxypropyl starch, methylhydroxyethyl starch, and the like; alkali metal and ammonium salts of casein, such as ammonium casein-ate, sodium caseina-te, lithium caseinate, potassium caseinate, and the like; alkali metal and ammonium salts of lignosulfonic acid; alkali metal and ammonium salts of naphthalene sulfonic acid; alkali metal and ammonium salts of glucuronic, galacturonic, mannuronic and related acid structures present in various vegetable mucilages, pectins, and gums, and the like; polyoxyalkylene glycols, such as polyoxyethylene glycol, polyoxypropylene glycol, and the like; adducts of ethylene oxide and nonylphenol, partially hydrolyzed poly(vinyl acetate), poly (vinylpyrollidine) and the like.

In preferred embodiments of this invention it is desirable also to include some plasticizer for the nitrocellulose in the aqueous dispersions in the weight ratio of from about 110.2 to 1:1 parts nitrocellulose to plasticizer, and for these preferred embodiments I employ the dispersions of my copending application Serial No. 391,321 in which at least a major portion of the plasticizer exists in a phase separate from the nitrocellulose phase of the dispersion. This is necessary in order to apply the dispersions to paper at the size press of the papermaking machine and then carry the coated paper over the hot drying cans of the machine without sticking. In making these preferred dispersions, it is possible to use either a water-soluble plasticizer or a water-insoluble plasticizer for nitrocellulose, and any of the vast variety of nitrocellulose plasticizers with which the art is acquainted may be employed. Examples of water-soluble plasticizers are glyceryl monoaceta-te, glyceryl diacetate, tris(tetrahydrofurfuryl) phosphate, triethylene glycol monoacet-ate, triethylene glycol diacetate, and the like. Examples of water-insoluble plasticizers are dialkyland dicycloalkyl .phthalates, such as dibutylphth-alate, dioctylphthalate, dicyclohexylphthalate and the like; trihydrocarbon phosphates, such as tricresyl phosphate, trioctyl phosphate, triphenyl phosphate, dioctyl phenyl phosphate, and the like; and miscellaneous other plasticizers, such as tributyl aconitate, acetyl tributyl citrate, di-butyl sebacate, diethylene :glycol monolaurate, and the like.

When the plasticizer is water-soluble, it can be added as such to the aqueous dispersion of nitrocellulose particles. However, when the plasticizer is water-insoluble, it is necessary to pre-emulsify it in water and then mix the plasticized emulsion with the nitrocellulose dispersion. In the case of either type plasticizer, the nitrocellulose particles in the dispersions become at most only partially plasticized. That is, the major amount of the plasticizer is not absorbed into the nitrocellulose particles but remains in a separate phase. Thus, the bulk of a watersoluble plasticizer remains dissolved in the aqueous phase of the dispersion whereas the water-insoluble plasticizer exists in a separate dispersed phase.

In some cases it is desirable that there be a partial plasticiza-tion of the nitrocellulose particles, and, in fact, partial plasticiz'ation does take place whenever a watersoluble plasticizer is employed. By this it is meant that a portion of the plasticizer is absorbed by the nitrocellulose. This partial absorption of plasticizer is accompanied by some colloiding action on the nitrocellulose particles causing them to assume a more rounded shape and become more densified. The same type of partial plasticization accompanied by densification of the nitrocellulose particles can be achieved also with a water-insoluble plasticizer by heating the dispersion composition to a temperature of about 85 C. to 100 C. for a period of about 1 minute-or more. Such partial plasticization 6 of the nitrocellulose particles is especially advantageous for dispersions in which some of the nitrocellulose particles are in the form of needle-shaped fibrils that range up to about 8 to 15 microns in length and which cause the dispersion, undesirably, to have a greater viscosity than otherwise.

In addition to plasticizer, it is desirable to include in some dispersions an emulsion of wax, e.g., paraffin wax, to provide added assurance that the composition can be processed trouble-free on a papermaking machine and to enhance the water resistance of the ultimate coating. In other cases it is desirable to include in the dispersion composition a resin of the type'commonly employed in nitrocellulose lacquers for the purpose of improving the degree of film build-up and to promote better depth and gloss in the final coating. The resin, when it is used, will normally be incorporated in the dispersion in the form of an aqueous emulsion. Pigments, dyes, starch, clays, and other additives can also be included in the composition.

The fluid dispersions of nitrocellulose particles in aqueous medium, with or without modifying additives, may be applied to any cellulosic substrate in sheet, fabric, or film form, such as, for example, kraft paper, glassine paper, rag stock paper, paperboard and the like, woven and knitted cotton fabric, linen fabric, ramie fabric, rayon fabric, book cloth, and the like, regenerated cellulose film such as viscose film, cellophane film, and the like, in any desired weight of coating, dry basis, in excess of about 0.8 pound per 3,000 sq. ft. Coating weights less than about 0.8 pound per 3,000 sq. ft. are generally too thin to provide a useful protective and/or decorative coating and too thin to be satisfactorily heat sealed.

The applied wet coatings of' the fluid dispersions of nitrocellulose particles in aqueous medium may be dried by a variety of conventional means well known in the art for evaporating volatile ingredients from coatings of nitrocellulose lacquers, lacquer emulsions, etc. For example, the wet coatings may be air dried at room temperatures, or at any convenient elevated temperature below the decomposition temperature of nitrocellulose by means of infrared heaters, circulated warm air ovens, drying tunnels or equivalent drying chambers, passage over heated rollers, and the like. In a papermaking machine, the paper coated with an aqueous dispersion of nitrocellulose particles in the size press of the papermaking machine, is dried by passage over the heated drying cans of the papermaking machine, augmented, if desire-d, by infrared heaters disposed as space heaters between the size press and drying cans of the papermaking machine.

An important and necessary element of the present invention involves the subsequent treatment of the dried coatings of nitrocellulose with a separate application of nitrocellulose plasticizer to impart improved thermoplasticity, film continuity, and heat sealing properties to the nitrocellulose coating. This subsequent treatment of the dried coatings is herein referred to as activation of the coating, and is uniquely beneficial for improving thermoplasticity, film continuity, and heat sealing properties to the nitrocellulose formulations of this invention which contain discrete particles of nitrocellulose. In one embodiment of the invention this activation may be accomplished on a papermaking machine by application of a Water solution of a water-soluble nitrocellulose plasticizer, such as diacetin, to the uncoated side of the coated paper at any convenient point between the drying cans of the papermaking machine and the calender rolls thereof. The water solution of the water-soluble nitrocellulose plasticizer should contain at least about 5%, preferably from about 10% to about 25%, of plasticizer by weight, and may be applied by spraying or by roller coating. Subsequent passage of the thus treated coated paper through the hot calender rolls of the papermaking machine effects evaporation of the Water and facilitates penetration of the plasticizer through the paper substrate and into the applied nitrocellulose coating to effect colloidization of the nitrocellulose to improve thermoplasticity, film continuity, and development of improved heatseal strength in the nitrocellulose coating.

In other embodiments of the invention this subsequent treatment of the dried coatings of nitrocellulose with a separate application of nitrocellulose plasticizer to activate the coating may also be accomplished on a papermaking machine by application of the plasticizer directly onto the dried nitrocellulose coating at any convenient point between the drying cans of the papermaking machine and the calender rolls thereof. A preferred point for such application is at the water box of the calender stack of a papermaking machine. The plasticizer employed for direct application to the dried nitrocellulose coating in these embodiments may be either a watersoluble nitrocellulose plasticizer such as diacetin, or a water-soluble nitrocelluose plasticizer such as dibutyl phthalate. In these embodiments the plasticizer may be applied as a water solution of a water-soluble plasticizer, as an emulsion or dispersion of a water-insoluble plasticizer in water, or as straight 100% plasticizer. Subsequent passage of the thus treated coated paper through the hot calender rolls of the papermaking machine, or otherwise subjecting the thus treated coated paper to an elevated temperature below the decomposition temperature of nitrocellulose for a short time, effects evaporation of water from -a water solution or emulsion of plasticizer in water and/or facilities penetration of the plasticizer into the applied nitrocellulose coating to improve thermoplasticity, film continuity and development of improved heat-seal strength in the nitrocellulose coating.

In still other embodiments of the invention in which any cellulosic substrate to which a fluid dispersion of finely-divided nitrocellulose in aqueous medium has been applied by any conventional coating means and subsequently dried, the subsequent treatment of the dried coatings of nitrocellulose with a separate application of nitrocellulose plasticizer to activate the coating is accomplished by application of the plasticizer directly onto the dried nitrocellulose coating. In these embodiments, also, the plasticizer may be either a water-soluble nitrocellulose plasticizer such as diacetin, or a water-insoluble plasticizer such as dibutyl phthalate, and may be applied as a Water solution of Water-soluble plasticizer, as an emulsion of a Water-insoluble plasticizer in water, or as straight 100% plasticizer. Subsequent heating of the thus treated coated cellulosic substrate for a short period, with or without application of pressure such as is provided by calender rolls and the like, effects evaporation of water from a water solution or emulsion of plasticizer in water and/or facilitates penetration and blending of the applied plasticizer into the applied nitrocellulose coating to improve thermoplasticity, film continuity and development of improved heat-seal strength in the nitrocellulose coating.

In any embodiment of this invention, the amount of plasticizer employed for separate application to the dried nitrocellulose coatings to activate the coatings generally will be between about 0.1 pound and about 0.6 pound per 3,000 sq. ft. of applied coating, preferably between about 0.2 pound and about 0.4 pound and sufficient to impart improved thermoplasticity, film continuity, and improved heat-seal strength to the nitrocellulose coating. It will be apparent, of course, that the amount of plasticizer which is employed in any particular case for this purpose depends somewhat on the amount of nitrocellulose plasticizer which may already be present in the applied dried nitrocellulose coating as a preplasticizing additive in the fluid dispersion of nitrocellulose particles in aqueous medium employed to coat the cellulosic substrate. Generally, the greater the amount of such preplasticizing additive which is present in the applied dried nitrocellulose coating, the lesser is the amount of plasticizer employed subsequently as a separate application to the dried nitrocellulose coating. To a lesser extent the amount of such separate application of plasticizer to the dried nitrocellulose coating also depends somewhat on the weight of nitrocellulose coating applied per unit of area of the cellulosic substrate, the greater the weight of nitrocellulose coating per unit area, the greater is the weight of separate application of plasticizer required to impart improved thermoplasticity, film continuity, and improved heat-seal strength to the nitrocellulose coating. Amounts of additionally applied plasticizer less than about 0.1 pound per 3,000 sq. ft. of nitrocellulose coating are generally insuflicient to impart significant improvement to the nitrocellulose coating, and amounts in excess of about 0.6 pound per 3,000 sq. ft. of nitrocellulose coating tend to make the nitrocellulose coating too soft to be useful. Any of the vast variety of solvent type nitrocellulose plasticizers, with which the art is acquainted, may be employed.

The present invention has also been found to be very effective and practical for activating nitrocellulose paper coatings of high pigment or filler content. More specifically, the cohesive and adhesive bonding strength in clay coatings on paper, wherein part or all of the binder content of such coatings employed to bond the clay particles together and bond the coating to the paper is derived from fluid dispersions of finely-divided nitrocellulose in aqueous medium, is greatly improved by activating such coatings in accordance with this invention, thereby greatly improving such properties as wet-rub resistance and ink pick resistance. When water-soluble nitrocellulose plasticizers such as diacetin, triacetin, tris(tetrahydrofurfuryl) phosphate, and the like, are employed for activating such clay coatings, it is desirable to apply them to the dried coating in the form of a thickened aqueous solution of the plasticizer, in which solution a watersoluble or dispersible thickening agent such as a gelatinized starch or starch derivative like oxidized starch is dissolved or dispersed to effect thickening.

A water-insoluble nitrocellulose plasticizer, such as dibutylphthalate may also be employed to activate such clay coatings, either in the form of a thickened aqueous emulsion of the plasticizer, employing a water-soluble or dispersible thickening agent such as a gelatinized starch or starch derivative like oxidized starch to effect thickening of the emulsion, or as a syrupy solution of nitrocellulose dissolved in the plasticizer. The purpose of the thickening agent in any of the above embodiments is to control and regulate the penetration of the activating plasticizer into the clay coating, and prevent rapid strikethrough of the activating plasticizer into the paper substrate beneath the clay coating.

The general nature of the invention has been set forth and the following examples are presented as specific illustrations thereof. All parts and percentages are by weight unless otherwise stated.

Example 1 A 20% by weight slurry of fibrous nitrocellulose in water, 11.5% nitrogen by weight, 56 seconds ASTM inch falling ball viscosity, was ground in a vertical paddle stirred ball mill with 4 inch diameter ceramic balls (the Szegvari Attritor Grinding Mill, manufactured :by The Union Processing Co., 118 Ash St., Akron, Ohio), for 6 hours. At this point, substantially all of the nitrocellulose fibers had been reduced to tiny irregular fragments, a substantial majority of which had a particle size in the range of about 2 to 3 microns, with a small percentage of agglomerates, platelets and needle-shaped particles up to about 15 microns. One percent (1%) by weight, based on dry nitrocellulose weight, of low viscosity 30% hydrolyzed poly(vinyl acetate) was then added to the Water slurry of nitrocellulose particles in the ball mill, and grinding was continued for another hour, whereupon all of the agglomerates and platelets were broken down to fine particles of 2-3 microns, with only a small percentage of needle-shaped fragments up to about 8 microns in length remaining. The resulting slurry of finely divided nitrocellulose particles was then concentrated to 40% by weight of nitrocellulose on a vacuum filter to produce a stiff, nonfiowing moist filter cake. The filter cake was then defiocculated by thoroughly blending thereinto 5.29% by weight, based on filter cake weight, of 30% partially hydrolyzed poly(vinyl acetate), thereby producing a smooth, homogeneous fluid dispersion of finely divided nitrocellulose particles in the water present in the moist filter cake.

The fibrous nitrocellulose employed in this example was in the form of relatively uniformly sized fiber aggregate particles obtained by nitrating cellulose fiber aggregate particles prepared by cutting sheets of pulpboard into particles approximately A; inch x inch x inch in dimension. These cellulose fiber aggregate particles were not materially changed in physical form or dimension during nitration thereof.

The ASTM 3 inch falling ball viscosity characteristic of the nitrocellulose was measured on a 12.2% by weight solution of the nitrocellulose in a solvent composed of 55% toluene, 25% denatured ethyl alcohol, and 20% ethyl acetate by weight at 25? C., noting the time in seconds for a inch steel ball to fall freely ten inches through the solution.

The above fluid dispersion of finely divided nitrocellulose particles in water was employed to prepare a disperse-type hydrosol paper coating composition, as follows:

Thirty-three (33) pounds of dicyclohexylphthalate, 1.1 pounds of oleic acid, 1.1 pounds of paraffin wax and 0.33 pound of sorbitan monolaurate (Span 20) were heated to 95 C. and vigorously agitated with 8.8 pounds of water heated to approximately 90-95 C. to prepare a water-in-oil type of emulsion. T 0 this emulsion, while still hot, was added 44 pounds of a hot 50% by weight solution in water of a balsamic alkyd resin of terpinene maleic anhydride having an acid number of approximately 30 neutralized with ammonium hydroxide, and containing 0.77 pound of polyoxyethylene sorbitan monolaurate (Tween 20), 4.4 pounds of 30% partially hydrolyzed poly(vinyl acetate) and 0.22 pound of concentrated ammonia, thus producing an oil-in-water type of emulsion. This hot emulsion was added with agitation to 145.3 pounds of the above fluid dispersion of finelydivided nitrocellulose particles in water to produce a dispersetype hydrosol paper coating composition which was then heated in a jacketed vessel to 90 C., and then cooled. While cooling, 10 grams of an aqueous emulsion of silica and mineral oil as a defoaming agent, 10 grams of a fungicide (para-chlorometa-xylenol), and 53 grams of magnesium montmorillonite gelant (Ben-A-Gel, E. W. Baroid Div., National Lead Co.) were added. The completed coating composition had a total solids of 53% by weight, and a Brookfield viscosity at C. of 100 centipoises.

The above disperse-type hydrosol paper coating composition, diluted with water to 44% total solids, and subsequently to 38% total solids, was applied at both dilutions onto 25 pound basis weight bleached kraft paper at 700 feet per minute by means of a Beloit gate-roll coater (Beloit Iron Works, Beloit, Wis.). The Beloit gate-roll coater is an inclined size press modified with metering rolls which apply the coating, or sizing material by transfer onto one or both of the size press rolls, and the coating material on the size press roll is then transferred to the paper in the nip between the size press rolls. The coated paper was then carried over heated drying drums (cans), augmented with infrared space heaters and a current of heated air, in the draw between the gateroll coater and the drying drums, to dry the applied coating. By this means a series of coated papers was thus obtained with coating weights of 2.3, 1.5 and 1.0 pounds per ream of 3,000 sq. ft. When tested for heat-sealing at 300 F. and 60 p.s.i. for /2 second, the 2.3 pound coat- 10 ing gave excellent bonding, or paper tear in pulling apart the heat-sealed joint. The 1.5 pound coating gave fair bonding, but no paper tear i.e., the sealed joint could be pulled apart without tearing the paper. The 1.0 pound coating gave very weak heat-sealed joints.

The coated papers were then activated by applying approximately 0.2 pound of butyl benzyl phthalate per ream of 3,000 sq. ft. directly onto the dried coatings in the nip between a pair of rotating rubber covered rolls. To accomplish this a thin layer of the plasticizer was metered onto the surface of the rubber covered roll which contacted the applied dried coating, and the plasticizer was transferred from the surface of the roll directly onto the dried coating as the coated paper passed through the nip between the rotating rolls. All of the thus activated coated papers gave excellent paper tear heat-seals at 300 F. and 60 p.s.i. for V2 second, including the paper with only 1 lb. of coating per ream.

Example 2 A disperse-type hydrosol paper coating composition was prepared as described in Example 1, and was then diluted with water to 50% by weight solids content. Urea was then dissolved in the aqueous phase of the coating composition to the extent of 4.5 pounds of urea for each 35.6 pounds of the 30% solids coating composition, thus raising total solids to 55.5% by weight. The urea was added to serve as plasticizer for the paper. The urea modified coating composition was used to coat 25 pound basis Weight bleached kraft paper at the size press of a papermaking machine. The coated paper was dried over the heated drying cans of the papermaking machine, and machine calendered. During part of the coating run, the paper was activated by applying approximately 0.2 pound of butyl benzyl phthalate per ream directly onto the dried coating at the calender stack of the papermaking machine. This was done by metering the plasticizer onto the face of one of the steel rolls in the paper machine calender, and the plasticizer was then transferred to the coated side of the paper when it passed through the nip. Coated paper was thus obtained with a range of coating weights of 1.4 to 2.2 pounds per ream. These papers were heat-sealed at 275 F., and also at 300 F., for /2 second at 60 p.s.i.

The unactivated coated papers all gave weak bonding at 275 F., and fair to good bonding at 300 F., but without paper tear. All of the activated coated papers gave excellent paper tear heat-sealed bonds at both 275 F. and at 300 F.

Example 3 Example 2 was duplicated with the exception that triacetin was employed to activate the coated paper in place of the butyl benzyl phthalate employed in Example 2 for activation. Substantially the same excellent paper tear heat-seal bonds at both 275 F. and 300 F. were obtained with triacetin as were obtained with butyl benzyl phthalate.

Example 4 Example 2 was duplicated with the exception that butyl phthalyl butyl glycolate was empoyed to activate the coated paper in place of the butyl benzyl phthalate employed in Example 2 for activation. Substantially the same excellent paper tear heat-seal bonds at both 275 F. and at 300 F. were obtained with butyl phthalyl butyl glycolate as were obtained with butyl benzyl phthalate.

Example 5 A 20% by weight slurry of fibrous nitrocellulose in water, 11.5% nitrogen by weight, 5-6 seconds ASTM inch falling ball viscosity, was ground in a vertical paddle stirred ball mill with A inch diameter ceramic balls (the Szegvari Attritor Grinding Mill) for 11 hours. Two percent (2%) by weight, based on dry nitrocellulose 1 1 Weight, of 88% hydrolyzed poly(vinyl acetate) was then added to the water slurry of nitrocellulose particles in the ball mill, and grinding was continued for another hour, whereupon substantially all of the nitrocellulose fibers had been reduced to tiny irregular fragments having particles sizes in the range of about 2-3 microns. The resulting slurry of finely divided nitrocellulose particles was then drained and washed on a vacuum filter with deionized water to produce a stiff, nonfiowing moist filter cake containing 40% by weight of nitrocellulose. The filter cake was then deflocculated by thoroughly blending thereinto 17.0 pounds of a 26% by weight solution in water of 88% hydrolyzed poly(vinyl acetate) (4.4 pound, dry weight) for each 125.5 pounds of moist filter cake, thereby producing a smooth, homogeneous fluid dispersion finely divided nitrocellulose particles in the ater present in the moist filter cake.

The fibrous nitrocellulose employed in this example was in the form of relatively uniformly sized fiber aggregate particles obtained by nitrating cellulose fiber aggregate particles prepared by cutting sheet of pulp board into particles approximately inch x inch x V 2 inch in'dimension. These cellulose fiber aggregate particles were not materially changed in physical form or dimension during nitration thereof.

The above fluid dispersion of finely divided nitro-cellulose particles in water was employed to prepare a dispersetype hydrosol paper coating composition, as follows:

Fifty and five-tenths (50.5) pounds of a 50% by weight dispersion of terpinene-maleic anhydride alkyd resin in dilute ammonia was mixed with 1.26 pounds of oleic acid, 12 pounds of water, and an additional 048 pound of concentrated ammonia. This was heated to 75 C. A solution of 37.7 pounds of dicyclohexyl phthalate, 0.88 pound of ethylene oxide adduct of sorbitan monolaurate, 0.38 pound of sorbitan monolaurate, and 3.8 pounds of pentaerythritol tetrastearate was heated to 100 C. and emulsified in the above aqueous dispersion of resin. To the hot emulsion was added with stirring 142.5 pounds of the above fluid dispersion of finely divided nitrocellulose particles in water. At first the emulsion thickened with the addition of part of the nitrocellulose dispersion, and then suddenly became quite fluid as more of the aqueous dispersion was added and the emulsion inverted. After adding all of the nitrocellulose dispersion, the temperature was 68 C. It was heated further to 74 C., then cooled. Thirty-one hundredths (0.31) pound of bentonite gelant was added to prevent caking. The resulting hydrosol paper coating composition had a total nonvolatile concentration of 52.5% by weight.

This was heated with live steam to 150 F. and was thus diluted with condensate to 35% solids by weight. It was applied to 25 pound basis weight bleached kraft paper at the size press of a paper machine, under difierent conditions of nip pressure, and was dried without sticking to the drying cans. Dry coating weights were obtained that varied from 0.8 to 1.4 pounds per ream of 3,000 square feet of paper. The calendered paper with the 1.4 pound coating gave very good peel strength when heat-sealed at 300 F., 60 p.s.i. sealing bar pressure for /2 second dwell, but no paper tear., i.e., the sealed joint could be pulled apart without tearing the paper. The two lower coating weight papers gave poorer peel strength.

The coated papers were then activated by applying approximately 0.6 pound per ream of 3,000 square feet of diacetin in the form of a by weight solution in water to the uncoated side of the paper by spraying, and supercalendering at 180 F. All of the thus activated coated papers gave excellent paper tear heat-seals at 300 F. and 60 p.s.i. for /2 second, including the paper with only 0.8 pound of coating per ream.

Example 6 Two thousand five hundred (2,500) parts, dry-weight of water-wet nitrocellulose, 12% nitrogen by weight, 3-4

seconds ASTM 7 inch falling ball viscosity, were dissolved in a solvent mixture consisting of 4,250 parts ethyl acetate and 4,250 parts toluene. This solution was then agitated with 10,000 parts of water containing 216 parts of a 58% by weight solution of Alipal CO-436 (ammonium salt of sulfate ester of ethylene oxide condensate of alkylphenol) (Antara Chemical Co.,) to form a crude lacquer-in-water emulsion which was then homogenized by passing it twice through a Manton-Gaulin homogenizer at 3,500 p.s.i. Solvent was then stripped from the emulsion by vacuum evaporation at 5560 C. and reduced pressure, employing a vacuum at 75-80 mm. of mercury at the beginning of the stripping operation and increasing the vacuum to about 20 mm. of mercury at the end of the stripping operation. During this operation the nitrocellulose was precipitated from the emulsified lacquer globules to form a dispersion of finely divided nitrocellulose particles in the water of the emulsion, and this dispersion was concentrated to 46% by weight of nonvolatiles during the stripping operation. The finely divided nitrocellulose particles obtained in this way were substantially all spheres and spheroids ranging in size from sub-micron size to about 6 microns maximum, with a substantial majority of the particles being less than 5 microns in diameter.

The above fluid dispersion of finely divided nitrocellulose particles in water was employed to prepare a disperse-type hydrosol paper coating composition, as follows:

Thirty (30) parts of crystalline dicyclohexyl phthalate and 0.3 part of sorbitan monolaurate (Span 20) were heated together to C. and then vigorously agitated with 8 parts of water heated to approximately 9095 C. to form a water-in-oil type of emulsion. To this emulsion, while still hot, was added slowly 15 .parts of an aqueous solution containing 1.2 parts of 88% hydrolyzed poly (vinyl acetate) and 0.7 part of polyoxyethylene sorbitan monolaurate (Tween 20) to convert the emulsion to an oil-in-water type of emulsion which was then mixed with 87 parts of the above fluid dispersion of finely divided nitrocellulose particles containing 36.6 parts of nitrocellulose, and also containing 1.2 parts of 88% hydrolyzed poly(vinyl acetate), added to the dispersion to retard settling. There was thus produced a disperse-type hydrosol paper coating composition containing 52% by weight of solids and having a Brookfield viscosity of 160 cps. at room temperature.

This composition was applied by means of the trailing blade coater to 25 pound basis weight of bleached kraft paper, and dried, to give a coating weight of 2.3 pounds per 3,000 square feet. Part of this coated paper was activated by applying to the coated surface a uniform layer of 0.2 pound per 3,000 square feet of butyl benzyl phthalate. Both the activated and the unactivated coated papers were tested for heat-sealing after drying, but without calendering. At 300 F. and 60 p.s.i. for /2 second, the unactivated coated paper gave very Weak heat-sealed joints, whereas the activated coated paper gave excellent paper tear heat-sealed joints.

Example 7 Approximately 20 parts of a moist filter cake containing 8 parts by weight dry basis of finely divided nitrocellulose particles, produced by grinding fibrous nitrocellulose, 11.5% nitrogen by weight, 5-6 seconds ASTM 7 inch falling ball viscosity, in a vertical paddle stirred ball mill and draining on a vacuum filter, as described in Example 1, was defiocculated by mixing with approximately 52 parts of a water solution containing 8 parts by weight, dry basis, of freshly cooked oxidized starch to give 72 parts, total weight, of fluid dispersion. This was mixed with a slurry of parts of kaolin in 60 parts of water to produce 232 parts of a clay filled paper coating composition containing 50% by weight total solids, and having 16 parts of binder per 100 parts of clay. This composition had a Brookfield viscosity at 60 r.p.m. of 180 centipoises at 23 C. This composition was used to coat offset paper by means of the air knife coater, and the coated paper was dried and calendered to give a coating weight of 10.2 pounds per ream of 3,000 square feet. This coated paper had very poor wet-rub resistance and showed ink pick resistance corresponding to initial picking with No. 3 viscosity ink at a speed of 205 feet per minute.

Replicate sheets of the same clay-coated paper were activated by applying a water solution containing 10% by weight of freshly cooked oxidized starch, 5% by weight of triacetin, and by weight of tris(tetrahydrofurfuryl) phosphate by hand draw-down using a No. 3 Mayer rod. The activated coated paper was then dried and calendered. A total activated coating weight of about 12 pounds per ream was thus obtained, which had excellent wet-rub resistance and an improved ink pick resistance corresponding to initial picking with No. 6 viscosity ink at 205 feet per minute.

Wet-rub resistance was determined by rubbing a freshly wetted index finger lightly over the coating three times and immediately transferring any clay thus picked up by the finger to dull black paper. The resistance was rated from excellent to very poor, depending upon the amount of clay transferred to the black paper; a rating of excellent denoting no transfer of clay, and a rating of very poor denoting an opaque white smear transferred.

The ink pick resistance was determined by the ink pick test described in TAPPI Journal, volume 34, No. 10, October 1951, pp. 433-438 by E. 1. Barber and J. W. Davis. Briefly, the test consists of uniformly distributing printing inks of different standard viscosities on a narrow printing roll which can be rotated at diiferent controlled speeds. While being driven at a controlled speed, the roll is brought into contact with a coated paper. The higher the viscosity of the ink and the greater the speed of the printing roll that the coating will withstand without picking or flaking, the greater is the adhesive bonding strength of the coating to the paper base stock and the cohesive bonding strength within the coating.

Example 8 Approximately 10 parts of a moist filter cake containing 4 parts by weight, dry basis, of finely divided nitrocellulose particles, produced by grinding fibrous nitrocellulose, 11.5% nitrogen by weight, 5-6 seconds ASTM inch falling ball viscosity, in a vertical paddle stirred ball mill and draining on a vacuum filter, as described in Example 1, was deflocculated by mixing with approximately 28 parts of a water solution containing 4 parts by weight, dry basis, of freshly cooked oxidized starch. To this was then mixed an additional 36 parts of a water solution containing 8 parts by weight, dry basis, of freshly cooked oxidized starch to give 72 parts, total weight, of fluid dispersion. This was then mixed with a slurry of 100 parts of kaolin in 60 parts of water to produce 232 parts of a clay filled paper coating composition containing 50% by weight total solids, and having 16 parts of binder per 100 parts of clay. This composition had a Brookfield viscosity at 60 r.p.m. of 315 centipoises at 28 C. This composition was used to coat offset paper by means of the air-knife coater, and the coated paper was dried and calendered to give a coating weight of 12.5 pounds per ream of 3,000 square feet. This coated paper had very poor wet-rub resistance and showed ink pick resistance corresponding to initial picking with No. 3 viscosity ink at a speed of 205 feet per minute.

Replicate sheets of the same clay coated paper were activated by applying a 5% solution by weight of RS A2 second nitrocellulose in dibutyl phthalate by hand drawdown using a No. 3 Mayer rod. The activated coated paper was then hot calendered. A total activated coating weight of about 13.5 pounds was thus obtained, which had excellent wet-rub resistance and an improved ink 14 pick resistance corresponding to initial picking with No. 6 viscosity ink at 285 feet per minute.

Example 9 Approximately 17.4 parts of the fluid dispersion of finely divided nitrocellulose particles in water described in Example 6, and containing 8 parts of nitrocellulose, were mixed with approximately 54.6 parts of a water solution containing 8 parts of freshly cooked oxidized starch to give 72 parts, total weight of fluid dispersion. This was mixed with a slurry of parts of kaolin in 60 parts of water to produce 232 parts of a clay filled paper coating composition containing 50% by weight total solids, and having 16 parts of binder per 100 parts of clay. This composition was used'to coat 50 pound basis weight offset paper by means of the air-knife coater, and the coated paper was dried and calendered to give a coating weight of 10.6 pounds per ream. This coated paper had very poor wet-rub resistance and showed ink pick resistance corresponding to initial picking with No. 3 viscosity ink at a speed of 205 feet per minute.

A replicate sheet of the same clay coated paper was activated by applying a water solution containing 8.2% by weight of freshly cooked oxidized starch and 18% by weight of diacetin by hand draw-down using a No. 3 Mayer rod. The activated coated paper was then dried and calendered. A total activated coating weight of about 12.9 pounds per ream was thus obtained, which had excellent wet-rub resistance and an improved ink pick resistance corresponding to initial picking with No. 6 viscosity ink at 97 feet per minute.

What I claim and desire to protect by Letters Patent is:

1. The process which comprises applying a coating of a fluid dispersion of nitrocellulose particles in aqueous medium to a cellulosic substrate, drying the applied coating, and thereafter activating the applied dried coating by treating said coating with a separate application of nitrocellulose plasticizer, said plasticizer being applied from aqueous medium.

2. The process in accordance with claim 1 in which the amount of said separate application of nitrocellulose plasticizer is between about 0.1 pound and about 0.6 pound of plasticizer per 3,000 square feet of applied dried coating, and sufficient to impart improved thermoplasticity, film continuity, and improved heat-seal properties to the nitrocellulose coating.

3. The process in accordance with claim 1 in which the cellulosic substrate is paper, and in which the separate application of nitrocellulose plasticizer is applied directly on the dried coating as a water solution of a water-soluble plasticizer.

4. The process which comprises continuously applying a coating of fluid dispersion Olf nitrocellulose particles in aqueous medium to paper in the size press of a papermaking machine, continuously passing the thus coated paper over the heated drying cans of the papermaking machine to dry the applied coating and thereafter activating the dried coating by treating said coating with a separate application of nitrocellulose plasticizer, in aqueous medium to impart improved thermoplasticity, film continuity, and improved heat-seal strength to the nitrocellulose coating.

5. The process which comprises (1) continuously applying a coating of a fluid dispersion of nitrocellulose particles in aqueous medium to paper in the size press of a paper-making machine, (2) continuously passing the thus coated paper over the heated drying cans of the papermaking machine to dry the applied coating, (3) continuously applying a water solution of a water-soluble nitrocellulose plasticizer to the uncoated side of the paper in an amount sufficient to deposit between about 0.1 pound and about 0.6 pound of plasticizer per 3,000 sq. ft. of coated paper, and (4) thereafter continuously passing the thus plasticizer treated coated paper through the heated calender stack of the papermaking machine.

6. The process which comprises (1) continuously applying a coating of a fluid dispersion of nitrocellulose particles in aqueous medium to paper in the size press of a papermaking machine, (2) continuously passing the thus coated paper over the heated drying .cans of papermaking machine to dry the applied coating, (3) continuously applying, from aqueous medium, a thin coating of nitrocellulose plasticizer directly onto the dried coating in an amount between about 01 pound and about 0.6 pound of plasticizer per 3,000 sq. ft. of dried coating, and (4) thereafter continuously passing the thus plasticize-r treated coated paper through the heated calender stack of the papenmakin-g machine.

7. The process in accordance with claim 6 in which the p-lasticizer applied in step (3) is straight undiluted plasticizer.

8. The process in accordance with claim 6 in which the plasticizer applied instep (3) is a water solution of a Water-soluble nitrocellulose plasti-cizer in an amount sufficient to deposit between 0.1 pound and about 0.6 pound of plasticizer per 3,000 sq. ft. of dried coating.

9. The process which comprises applying a clay filled coating of a fluid dispersion of nitrocellulose particles in aqueous medium to paper, drying the applied coating, and

thereafter activating the applied dried coating by treating said coating with a separate application of nitrocellulose p'lasticizer to improve cohesive and adhesive bonding strength of the clay filled coating, said plasticizer being applied from aqueous medium.

10. The process in accordance with claim 9 in which the activating plasticizer which is applied to the dried coating is a thickened aqueous solution of a water-soluble nitrocellulose plasticizer containing an oxidized starch as thickening agent.

References Cited by the Examiner UNITED STATES PATENTS 1,631,750 6/1927 McIntosh 117157 X 1,848,268 9/1929 Persiel 106-170 X 2,722,528 11/1955 Johnson 106-198 2,843,582 7/1958 Voris 106170 X 2,843,583 7/1958 Voris 106-170 X 3,148,077 9/1964 Garetto 11763 X MURRAY KATZ, Primary Examiner.

H. W. MYLIUS, Assistant Examiner. 

1. THE PROCESS WHICH COMPRISES APPLYING A COATING OF A FLUID DISPERSION OF NITROCELLULOS PARTICLES IN AQUEOUS MEDIUM TO A CELLULOSIC SUBSTRATE, DRYING THE APPLIED COATING, THEREAFTER ACTIVATING THE APPLIED DRIED COATING BY TREATING SAID COATING WITH A SEPARATE APPLICATION OF NITROCELLULOSE PLASTICIZER, SAID PLASTICIZER BEING APPLIED FROM AQUEOUS MEDIUM. 